Cooking hobs with a glass-ceramic plate as cook top or cooking surface are common technology. Such glass-ceramic plates are usually available as flat plates or shaped three-dimensionally.
Glass-ceramics with high-quartz mixed crystals as prevailing crystalline phase are made of crystallisable 20 lithium aluminum silicate glasses.
The production of these glass-ceramics is effected in several steps.
Glass-ceramic is produced in several stages. The crystallization is started by a glass consisting of a mixture of broken pieces of glass and a powdered mixture of raw materials which is usually melted at temperatures between 1500 and 1650° C. During the melting process usually arsenic and/or antimony oxide are used as refining agents. These refining agents are compatible with the required glass-ceramic properties and lead to excellent blister qualities of the molten mass. Even though these substances are firmly embedded in the glass structure, they are still disadvantageous in view of safety and environmental protection aspects. Therefore, during the raw material extraction, raw material processing and because of the evaporation during the melt, special precautions must be obeyed.
Recently, particularly the application of SnO2 is described as harmless refining agent. In order to achieve perfect blister qualities at conventional melting temperatures (maximum approx. 1680° C.) besides SnO2, preferably halide compounds are used as additional refining agents. Thus, in Japanese applications JP 11 100 229 A and JP 11 100 230 A, the application of 0.1-2 weight % SnO2 and 0-1 weight % Cl is described. According to these documents the coloring is achieved by addition of V2O5 as sole colorant.
The addition of 0.05-1 weight % fluorine (US 2007 0004578 A1) or 0.01-1 weight % bromine (US 2008 0026927 A1) to support the refine by SnO2 is disclosed as well. Also in these documents refining temperatures below 1700° C. are described. The main coloring agent is V2O5. A disadvantage with the addition of halides as they are evaporating so highly during the melting temperature is that they are producing toxic compounds such as HF.
The application of SnO2 in relation with high-temperature refining above 1700° C. to achieve good blister quality is described in DE 199 39 787 C2. This document, however, provides no guidance for the achievement of good indication capability within the wavelength range beginning at 450 nm.